A number of advanced energy storage device technologies have recently been developed, such as metal hydride (e.g., Ni-MH), lithium-ion, and lithium polymer cell technologies, which promise to provide high energy generation for a wide range of commercial and consumer applications. In high-energy applications, a substantial number of individual energy storage devices or cells are typically connected in series to produce a requisite amount of electrical power. By way of example, it is believed that a battery system suitable for powering an electric vehicle will likely have a voltage rating on the order of several hundred volts, and a current rating on the order of several hundred amperes.
In applications in which rechargeable energy storage cells are connected together in series, it is considered desirable to use cells which are equivalent or very similar in terms of electrochemistry and voltage/current characteristics. It is known that undesirable consequences often result during charging and discharging when an energy storage cell within a series string of cells exhibits characteristics that vary significantly from those of other serially connected energy storage cells.
For example, the energy output of a series string of electrochemical cells is limited by the performance of individual cells within the series connection. A defective or anomalously operating cell reduces the overall performance of the series connected cells, yet attempts to operate at a level equivalent to that of higher performing cells in the series string. This undesirable imbalance in cell operating characteristics results in accelerated degradation of the poor performing cell which, in turn, increases the rate at which overall energy system performance degrades.
Another adverse consequence of cell asymmetry within a series connection involves the voltage of an anomalous energy storage cell within the series string, which will rapidly exceed a nominal maximum voltage limit during charging. Such an overvoltage or overcharge condition may damage the cell and significantly reduce the service life of the cell and other cells within the series connection.
A number of techniques have been developed to moderate the adverse consequences arising from the continued presence of a defective cell within a series string of cells. Such techniques, however, are typically inapplicable in high-current, high-voltage power generating systems. Other known implementations exhibit unacceptably high resistance to current flow through the series connection, thus reducing the power delivered by the series string of cells and increasing heat generation within the cell string. Such implementations typically exhibit undesirable leakage current characteristics as well.
It can be appreciated that the characteristics of mass manufactured energy storage cells will deviate to varying degrees from a given set of build requirements. Further, cell characteristics, even if considered acceptable at the time of manufacture, will deviate from manufactured specifications at varying rates and to varying degrees over time.
There is a need in the battery manufacturing industry for an apparatus and method for effectively and safely moderating the adverse impact of a defective cell on the overall performance of series connected energy storage cells. There exists a further need for such an implementation which is also capable of handling a large current flow through the series connection. The present invention fulfills these and other needs.